Steel Part and Method for Manufacturing the Same

ABSTRACT

A plurality of layers are laminated on at least part of the member under treatment made of steel, the plurality of layers having carbon concentrations higher than that of the member under treatment and 1.0 wt. % or less, the carbon concentration of an outermost layer of the plurality of layers being the highest. A method for manufacturing a steel part, including spraying powder containing carbon on at least part of an member under treatment made of steel so as to form a first layer having a carbon concentration higher than that of the member under treatment and spraying powder containing carbon on at least part of the first layer so as to form a second layer having a carbon concentration higher than that of the first layer. Carbon concentrations of a plurality of layers including the first layer and the second layer are 1.0 wt. % or less.

TECHNICAL FIELD

The present invention relates to a steel part and a method formanufacturing the same.

BACKGROUND ART

Since machine parts such as drive parts, gears, and bearings are alwaysexposed. to heavy loads, they need to have high mechanical strength suchas hardness and fatigue resistance. These machine parts are made fromsteels used for machine structures such as carbon steel, chromium.steel, chromium molybdenum steel, and nickel chromium-molybdenum steel.

The steels used for machine structures may need two opposite propertiesfor an outer portion and an inner par Lion, that is, a surface of thesteels needs high fatigue resistance and the material itself needs highfracture resistance to secure shock resistance of the part. The materialmay have a base material having a comparatively low carbon concentrationand a high fracture resistance, for example, a low alloy steel (such asSCM415 defined in JIS-G4104). The surface of the material is oftensolid-soluted with carbon so as to increase the carbon concentration,and carburizing treatment and carbonitriding treatment are oftenperformed to improve hardness and fatigue resistance.

However, when carbon is simply dispersed on the surface of the material,a surface hard layer has a gradient in carbon concentrationdistribution. Thus, it is difficult to form a thick composition having adesired carbon concentration. When the surface hard layer is thickened,the surface is excessively carburized (excessive carburization). As aresult, the surface hard layer embrittles. For example, PTL 1 disclosesa method for forming a hard film of high carbon steel or a high-carbonlow-alloy steel on a surface of a base material and thermallydiffuse-bonding the base material and the hard film so as to securedesired strength.

CITATION LIST Patent Literature

PTL 1: JP 11-222663 A.

SUMMARY OF INVENTION Technical Problem

However, according to the method disclosed in PTL 1, since there is alarge difference between the carbon concentration of the base materialas a member under treatment and that of the coating film of the highcarbon steel, the member under treatment and the surface hard layereasily peel off between the base material and the coating film, which isa problem of such a method.

An object of the present invention is to prevent the member undertreatment and the surface hard layer from peeling off.

Solution to Problem

To accomplish the foregoing object, configurations described for examplein the claims are used.

Advantageous Effects of Invention

According to the present invention, the member under treatment and thesurface hard layer can be prevented from peeling off.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an example of a block diagram of a steel part.

FIG. 2 is an example of a block diagram of the steel part.

FIG. 3 is an example of a process drawing showing a process for forminga first layer and a second layer on a member under treatment.

FIG. 4 is an example of a schematic diagram showing a steel partaccording to a second example.

FIG. 5 is an example of a schematic diagram showing a cross section ofthe steel part according to the second example.

FIG. 6 is an example of a process drawing showing a process for forminga steel part according to the second example.

DESCRIPTION OF EMBODIMENTS

A steel part according to the present invention includes a plurality oflayers having high carbon concentrations than that of a member undertreatment (high carbon steel layers) formed on the surface of the memberunder treatment (steel, part). The carbon concentrations of the highcarbon steel layers are 1.0 wt. % or less. The outermost layer has thehighest carbon concentration.

FIG. 1 shows an embodiment of the present invention. A first layer 102and a second layer 103 are successively laminated on at least part of amember under treatment 101 so as to form a steel part 100. The firstlayer 102 and the second layer 103 are high carbon steel layers havingcarbon concentrations of 1.0 wt. % or less. The carbon concentrations ofthe first layer 102 and the second layer 103 are higher than that of themember under treatment. The carbon concentration of the second layer ishigher than that of the first layer.

FIG. 2 shows another embodiment. According to the present embodiment, athird layer 104 is laminated on the second layer of the steel part shownin FIG. 1. The carbon concentration of the third layer 104 is higherthan that of the second laver. The carbon concentration of the thirdlayer 104 is 1.0 wt. % or less.

Examples of the member under treatment on which a surface treatment isperformed includes low carbon steel or low carbon alloy steel. Thematerials having high fracture resistance such as chromium steel,chromium molybdenum steel, chromium molybdenum nickel steel, chromiummanganese steel, and chromium nickel, steel (stainless steel) areexemplified, and alloy compositions thereof are defined in domestic andforeign standards such as JIS and ASTM.

The plurality of high carbon steel layers coating the member undertreatment is required to have carbon concentrations that are higher thanthat of the member under treatment and 1.0 wt. % or less. Since thecarbon concentrations of these layers are higher than that of the memberunder treatment, these layers have higher fatigue resistances. Inaddition, when the carbon concentrations of these layers are 1.0 wt. %or less, mesh-shaped cementite can be prevented from excessivelydeposited on a grain boundary. Thus, the surface can be prevented fromembrittling, and as a result, these high carbon steel layers become longfatigue-life layers. The high carbon steel lavers have at least thefirst layer and the second layer. When necessary, the high carbon steellayers also have one or more layers formed on the first and secondlayers. The carbon concentration increases from the member undertreatment toward the outermost layer.

To increase the carbon concentration of the steel part by coating themember under treatment with the layer without carburizing and diffusing,a plurality of layers is formed to decrease differences of carbonconcentrations between the layers, and therefore, peeling between themember under treatment and the high carbon steel layer and peeling,cracking and the like between the high carbon steel, layers can bereduced. In addition, the desired carbon concentrations and thicknessesof the layers can be freely adjusted. When the layer is a multi-layercomposed of three or more layers, even if the difference between thecarbon concentration of the outermost layer and the carbon concentrationof the member under treatment is large, since the differences of carbonconcentrations of individual layers can be decreased, peeling betweenthe member under treatment and the layer and peeling and the likebetween the layers can be reduced.

Although the drawing clearly separates layers, since carbon slightlydiffuses from a layer that has a high carbon concentration to a layerthat has a low carbon concentration, there is a gentle gradient ofcarbon concentrations at boundaries of layers in the film thicknessdirection. According to the present invention, a portion where there isa gradient of carbon concentrations is permitted as inter-layers(boundaries of layers).

Alloy compositions other than carbon of individual layers are notrestricted. Examples of these steel materials are high carbon steel andhigh carbon alloy steel. In particular, it is preferable that thecompositions of alloy elements of the first layer, the second layer, andat least one layer formed thereon if necessary (hereinafter these layersare referred to as the plurality of high carbon steel layers) are nearlythe same as that of the member under treatment, so that the high carbonsteel layers can be unified with the member under treatment and that aconjugated compound can be prevented from locally being formed.Alternatively, other alloy elements may be adjusted so as to improveproperties such as corrosive resistance and heat resistance other thanfatigue resistance, at the same time.

Although the plurality of high carbon steel layers may be formed on theentire surface of the member under treatment with equal thicknesses, theplurality of high carbon steel layers may have a thickness distributionon the surface of the member under treatment, and may be formed only onpart of the member under treatment. As a particular example from machineparts, each of the layers may be formed only at a portion that needsespecially high fatigue resistance such as a contact portion with abearing of a shaft, tooth of a gear, a contact portion with a member ofa press roll. These partial treatments are preferable when a portionthat needs fracture resistance and a portion that needs fatigueresistance are individually controlled. Further, each of the layers maybe formed on the surface of the member under treatment with distributionsuch as a portion having no layers, a portion having only the firstlayer, and a portion having both the first layer and the second layer.Likewise, at least one layer formed on the second layer may be formed onpart of the surface of the member under treatment.

The plurality of high carbon steel layers can be formed according tovarious methods. As methods that are excellent in forming speed andadherence to the member under treatment, a cold spraying method, a warmspraying method, a plasma spraying method, an arc spraying method, aflame spraying method, a building-up welding method, an aerosoldeposition method, and so forth are known. FIG. 3 shows a process forsuccessively forming individual layers.

(a) A member under treatment 101 is prepared to form high carbon steellayers.

A first layer 102 is formed on the member under treatment 101. Materialsof the individual layers are prepared in the form of powder, wires,rods, and the like, according to the individual methods. According tothe method shown in FIG. 3, powder is sprayed to be deposited on themember under treatment. For example, using powder having a low carbonconcentration (first powder), the layer is formed according to theforegoing method. The carbon concentration of the first powder is higherthan that of the member under treatment and 1.0 wt. % or less.Alternatively, the first powder may be a mixture of carbon and anotherpowder. When the layer is formed, the carbon concentration of the layercontained in the entire layer may be higher than that of the memberunder treatment and 1.0 wt. % or less.

(c) A second layer 102 is formed on the first layer 102. The secondlayer is formed on the first layer 102 using powder (second powder)having a higher carbon concentration than that of the first layer. Likethe first powder, the second. powder may be a mixture of carbon andanother powder. When the layer is formed, the carbon concentration ofthe entire layer may be higher than that of the first layer and 1.0 wt.% or less.

(d) As a result, a steel part 100 having two high carbon steel layers isformed.

Even if the high carbon steel layers is a multilayer, it is preferablethat the mixing ratios of a plurality of types of powders havingdifferent carbon concentrations are changed, because the kinds ofmaterials required for forming the layer. Although film formingconditions are appropriately adjusted depending on a method, a memberunder treatment, and materials of individual layers that are used, it ispreferable to form the layer at a temperature of the member undertreatment higher than the room temperature, because the film formingefficiency is improved, adherence of each of the layers to the memberunder treatment is improved, and mutual diffusion on the interface ofeach of the layers is accelerated. However, it is desirable that thefilm forming temperature is adjusted depending on various conditionssuch as heat resistances and oxidizing resistances of the materials ofthe member under treatment and the individual layers. After forming thelayers as described above, thermal treatment such as quenching andannealing and surface treatment such as carburizing and nitriding can beperformed.

Hereinbelow, examples will be described with reference to theaccompanying drawings.

EXAMPLE 1

In the present example, an example in which the steel part 100 is aplate will be described. FIG. 1 is a block diagram of the steel part 100in the present example. In the present example, the member undertreatment 101 used for the steel part 100 was a stainless steel platehaving a length of 50 mm, a width of 50 mm, and a. thickness of 10 mm(JIS standard: SUS 304, NISSHIN STEEL CO., LTD, 0.05 wt. %).

The first layer 102 and the second layer 103 were made from stainlesssteel powder (DAP304L, DAIDO STEEL LTD). Two types of powder, stainlesssteel powder A that is additive free and stainless steel powder B thatholds graphite powder of 2.0 wt. % (SIGMA-ALDRICH CO. LLC) were mixed atpredetermined weight ratios to prepare material powders of the firstlayer 102 and the second layer 103. In the present example, the materialpowders of the first layer 102 were mixed so that the carbonconcentration became 0.4 wt. % (stainless steel powder A: stainlesssteel powder B=8:2 (weight ratio) and the material powders of the secondlayer 103 were mixed so that the carbon concentration became 0.8 wt. %(stainless steel powder A: stainless steel powder B=6:4 (weight ratio),and the mixed material powders were uniformly maxed by a V-shaperotating mixer to prepare the material powders of each of the firstlayer 102 and the second layer 103. FIG. 1 is a schematic diagramshowing individual layers so that they can be easily distinguished.Table 1 lists real thicknesses of individual films that are formed.Theoretically, the carbon concentrations when the powder is adjustedmatches the carbon concentrations of the formed layers. However, sincecarbon is lost while materials are adjusted and layers are formed, thecarbon concentrations of the layers are slightly lower than those of thematerial powders.

FIG. 3 shows a process for forming the first layer and the second layerin the present example. The first layer and the second layer were formedby a cold spraying method under the condition that nitrogen gas was usedas carrier gas at a pressure of 4 MPa, the temperature of the memberunder treatment was 400° C., and the nozzle distance was 20 mm. When themember under treatment is heated, since powder easily adheres to themember under treatment, the adherence between layers further improves.Thereafter, material powders were changed and the second layer wasformed by the cold spraying method in the same conditions. Thereafter,the heat treatment was performed at 800° C. for 30 minutes so thatgraphite powder held on stainless steel powder B was solidified in eachof the layers and then quenching is performed at a speed of 100° C. persecond to form the steel part 100.

COMPARATIVE EXAMPLE 1

In the configuration of Example 1, a first layer having a carbonconcentration of 0.8 wt. % was formed by the cold spraying method, and asecond layer was not formed. The other forming conditions were the sameas those of Example 1.

In Example 1, also in a Falex test conducted with a test piece of around1 mm thick and Vickers hardness (Hv) of 920 on the surface of the steelpart 100, the surface treatment layer having good adherence withoutinter-laver peeling can be obtained. However, in Comparative Example 1,there was a problem in adherence due to small peeling between the memberunder treatment 101 and the first layer 102.

TABLE 1 EXAMPLE COMPARATIVE 1 EXAMPLE 1 FIRST THICKNESS (mm) 0.10 1.03LAYER AVERAGE CARBON 0.36 0.75 CONCENTRATION (wt. %) VICKERS HARDNESS710 910 (Hv) SECOND THICKNESS (mm) 1.01 LAYER AVERAGE CARBON 0.77CONCENTRATION (wt. %) VICKERS HARDNESS 920 (Hv) ADHERENCE BETWEEN BASENO CRACKING MATERIAL AND LAYER PEELING

EXAMPLE 2

In the present example, an example in which a steel part 100 is a shaftpart will be described. FIG. 4 is a schematic diagram of the steel part100. The steel cart 100 used a member under treatment 101 of chromemolybdenum steel (JIS standard.: SCM 415, DAIDO STEEL CO., LTD., 0.15wt. %) having a diameter of 30 mm and a length of 300 mm. A first layer102, a second layer 103, and a third layer 104 were formed at an endportion of the member under treatment.

FIG. 5 is a sectional view taken along a line A-A′ of FIG. 4. FIG. 5shows a cross section to a line C-C. The first layer 102, the secondlayer 103, and the third layer 104 were formed at the end portion of themember under treatment 101. All the three layers were formed within 100mm from the end portion, the first layer 101 and the second layer 102were formed at the portion between 100 mm and 110 mm from the endportion, and only the first layer 101 was formed at the portion between110 mm and 120 mm from the end portion of the member under treatment101. FIG. 4 is a schematic diagram showing individual layers so thatthey can be easily distinguished. Table 2 lists real thicknesses ofindividual films that are formed.

Each of these layers was made by mixing two types of powder, chromemolybdenum steel powder A and chrome molybdenum steel powder B atpredetermined weight ratios. Chrome molybdenum steel powder A is made ofchrome molybdenum steel powder (SCM 415, EPSON ATMIX CORPORATION) thathas the same composition as that of the member under treatment 101, andchrome molybdenum steel powder B is made by increasing only the carbonconcentration of the chrome molybdenum steel powder A to 2.0 wt. %, toprepare the material powders of these layers. In the present example,the material powders of the first layer 102 were mixed so that thecarbon concentration became 0.4 wt .% (chrome molybdenum steel powder A:chrome molybdenum steel powder B=86:14 (weight ratio)), the materialpowders of the second layer 103 were mixed so that the carbonconcentration became 0.6 wt. % (chrome molybdenum steel powder A: achrome molybdenum steel powder B=76.24 (weight ratio)), and the materialpowders of the third layer 103 were mixed so that the carbonconcentration became 0.8 wt. % (chrome molybdenum steel powder A: chromemolybdenum steel powder B=65:35 (weight ratio)), and the mixed materialpowders were uniformly mixed by a V-shape rotating mixer to prepare thematerial powders of these layers

FIG. 6 shows a process for forming the first layer, the second layer,and the third layer in the present example. Each of the layers wereformed using a plasma spraying method. The first layer, the secondlayer, and the third layer formed in this order by changing the materialpowders in the same conditions. In this configuration, since each of thelayers was partly formed, while the member under treatment 101pre-heated at 400° C. was being rotated, the member under treatment 101was scanned by a thermal spray nozzle 106 to form the individual layersat desired portions as shown in FIG. 6.

EXAMPLE 3

In the configuration of example 2, the material powders of the thirdlayer were prepared so that their carbon concentration became 1.0 wt. %(chrome molybdenum steel powder A: chrome molybdenum steel powderB=54:46 (weight ratio)) by the plasma spraying method. The other formingconditions were same as those of Example 2.

COMPARATIVE EXAMPLE 2

In the configuration of Example 2, the material powders of the firstlayer were prepared so that the carbon concentration became 0.8 wt. %(chrome molybdenum steel powder A: chrome molybdenum steel powderB=65:35 (weight ratio)) by the plasma spraying method, and the secondlayer and the third layer were not formed. The other forming conditionsof Comparative Example 2 were the same as those of Example 2.

COMPARATIVE EXAMPLE 3

In the configuration of Example 2, the material powders of the thirdlayer were prepared so that the carbon concentration became 1.1 wt. %(chrome molybdenum steel powder A: chrome molybdenum steel powderB=49:51 (weight ratio)) by the plasma spraying method. The other formingconditions of Comparative Example 3 were the same as those of Example 2.

The steel parts 100 obtained in Examples 2 and 3 and ComparativeExamples 2 and 3 were smoothened by a mechanical polishing method or abuffing method so that the surface roughness (Ra) became 1.0 μm or less.Thereafter, a Falex test was conducted in lubrication oil for afilm-forming portion of the third layer 103 based on ASTM-D-3233. Table2 shows the thicknesses of the individual layers measured by cutting thesteel parts 100 after the Falex test was confucted, average carbonconcentrations measured by an electron beam micro-analyzer (SHIMAZUCORPORATION), presence or absence of inter-layer peeling observed by anoptical microscope, surface roughness (Ra) of the outermost layers, andcross-sectional Vickers hardness measured by a micro Vickers hardnessmeter (SHIMAZU CORPORATION).

When the Falex test was conducted with an approximately 1-mm thick testpiece having a Vickers hardness (Hv) of 930 or higher on the surface ofsteel parts 100 in Examples 2 and 3, surface treatment layers havingexcellent adherence without inter-layer peeling were obtained. It wasconfirmed that in each of the surface treatment layers of each sample,mesh-shaped cementite was not deposited on a grain boundary and thelayers did not excessively carburize.

In contrast, in Comparative Example 2, there was a problem in adherencedue to small peeling between the member under treatment 101 and thefirst layer 102. Further, it was confirmed that in the conditions ofComparative Example 3, the surface roughness after the Falex test wasconducted was larger than that of the other steel parts, and the steelpart 100 was damaged by abrasion. As a result of an observation of thecomposition, it was confirmed that mesh-shaped cementite that wascharacteristic of hypereutectoid steel of a steel material was depositedon a grain boundary, and an carburized portion was damaged.

From each of the foregoing evaluations, it was confirmed that the steelparts having configurations disclosed in the present invention have asurface treatment layer having excellent surface hardness and excellentadherence to a member under treatment. Although a shaft as a machinepart has been described in this section, it is clear that theembodiments of the present invention can be applied to various machineparts such as drive parts, gears, and bearings.

TABLE 2 EXAMPLE EXAMPLE COMPARATIVE COMPARATIVE 2 3 EXAMPLE 2 EXAMPLE 3FIRST THICKNESS (mm) 0.10 0.10 1.03 0.08 LAYER AVERAGE CARBON 0.32 0.330.78 0.32 CONCENTRATION (wt. %) VICKERS HARDNESS 610 630 900 610 (Hv)SECOND THICKNESS (mm) 0.11 0.09 0.13 LAYER AVERAGE CARBON 0.55 0.57 0.53CONCENTRATION (wt. %) VICKERS HARDNESS 800 820 770 (Hv) THIRD THICKNESS(mm) 0.99 1.05 1.03 LAYER AVERAGE CARBON 0.77 0.99 1.05 CONCENTRATION(wt. %) VICKERS HARDNESS 940 930 960 (Hv) SURFACE ROUGHNESS (Ra, μm) 1.10.9 2.1 5.4 ADHERENCE BETWEEN BASE NO NO CRACKING NO MATERIAL AND LAYERPEELING PEELING PEELING

REFERENCE SIGNS LIST

-   100 steel part-   101 member under treatment-   102 first layer-   103 second layer-   104 third layer-   105 spray nozzle-   106 spray nozzle

1. A steel part having a member under treatment made of steel, aplurality of layers being laminated on at least part of the member undertreatment, the plurality of layers having carbon concentrations higherthan that of the member under treatment and 1.0 wt. % or less, thecarbon concentration of an outermost layer of the plurality of layersbeing the highest.
 2. The steel part according to claim 1, wherein theplurality of layers are three layers or more.
 3. The steel partaccording to claim 1, wherein the member under treatment has an area inwhich the plurality of layers are laminated and an area in which theplurality of layers are not laminated.
 4. The steel part according toclaim 1, wherein the plurality of layers includes a first layer and asecond layer laminated successively on the member under treatment, thefirst layer having an area in which the second layer is laminated and anarea in which the second layer is not laminated.
 5. A method formanufacturing a steel part, comprising: spraying powder containingcarbon on at least part of an member under treatment made of steel so asto form a first layer having a carbon concentration higher than that ofthe member under treatment; and spraying powder containing carbon on atleast part of the first layer so as to form a second layer having acarbon concentration higher than that of the first layer, wherein carbonconcentrations of a plurality of layers including the first layer andthe second layer are 1.0 wt. % or less.
 6. The method for manufacturingthe steel part according to claim 5, comprising: spraying powdercontaining carbon on at least part of the second layer so as to form athird layer having a carbon concentration higher than that of the secondlayer.
 7. The method for manufacturing the steel part according to claim5, wherein the plurality of layers are formed while the member undertreatment is heated.